High frequency immune squib



D. G. HOLINBECK 3,148,619

HIGH FREQUENCY IMMUNE SQUIB 9 Sheets-$heet 1 :na?g l z I r z z 1 z z w zz z z z a Sept. 15, 1964 Filed June 16, 1961 FIG. 2

JNVENTOR. DALE G. HOLINBECK Y E N R O T T A Sept. 15, 1964 FIG. 4

INVENTOR.

DALE G. HOLINBECK ATTORNEY p 5, 1964 D. G. HOLlNBECK HIGH FREQUENCYIMMUNE SQUIB 9 Sheets-Sheet 3 Filed June 16, 1961 FIG. 5

' INVENTOR. DALE G. ,HOLINBECK BY Q l IZJ ATTORNEY Sept. 15, 1964 D. G.HOLINBECZK 3,148,619

HIGH FREQUENCY IMMUNE SQUIB Filed June 16, 1961 9 Sheets-Sheet 4 FIG. 7

t c g" c as: c c 1..

FIG. 8

INVENT DALE G, HOLINBE ATTORNEY p 1964 n. e. HOLINBECK HIGH FREQUENCYIMMUNE SQUIB 9 Sheets-Sheet 5 Filed June 16, 1961 IIIIIIIIIIIIIIIJIIIIIIIIIIVIYII IIIIII llllllf FIG. LO

DALE G. HOLINBECK Q. Q

ATTORNEY Sept 15, 1964 Filed June 16, 1961 D. G. HOLINBECK HIGHFREQUENCY IMMUNE SQUIB 9 Sheets-Sheet 6 JQJM'J ATTORNEY p 15, 1964 D. e.HOLINBECK 3,148,619

HIGH FREQUENCY IMMUNE SQUIB Filed June 16, 1961 9 Sheets-Sheet 7 IIIIHIIlllllll llllll V out IIIIIH [0' lllllll l |||1|1| lkc lOkc iOO kc mcINVENTOR. DALE G. HOLINBECK FIG. \2 W Z Q ATTORNEY p 5, 1964 D. cs.HOLINBECK 3,148,619

HIGH FREQUENCY IMMUNE SQUIB Filed June 16, 1961 9 Sheets-Sheet 8 Volts 2FIG. .13

' INVENTOR. DALE G: HOLINBECK ATTORNEY Sept. 15, 1964 D. G. HOLINBECKHIGH FREQUENCY IMMUNE SQUIB 9 Sheets-Sheet 9 Filed June 16. 1961INVENTOR. DALE G. HOLINBECK 3V5 Q, QQJ

ATTORNEY 33433619 FREQUENCY IMMUNE QUII Daie G. I-Ioiinbecir, Madison,Wis, assignor to Bjorlrsten Research Laboratories for Indushy, Inc,Fitchburg, Wis, a corporation of Wisconsin Filed .In c 16, E953, er. No.117,531 13 Ciaims. (Ci. NEE-Q8) This invention relates to anelectrically actuated detonator for an explosive charge, hereaftercalled a squib, and more particularly relates to a squib in combinationwith circuitry which prevents the squib from being accidentallydetonated by high frequency current in the firing circuit.

Electrically detonated armaments such as rockets, missiles, andrecoilless rifles, and demolition charges such as dynamite arevulnerable to accidental detonation by high-intensity environmentalelectromagnetic radiation such as by radio and radar beams which induceelectric currents in the firing circuit of the armament. The problem ofaccidental detonation is particularly acute in the vicinity of radar andradio transmitters, and elaborate precautions are necessary for armingaircraft and the like to prevent accidental firing of the armaments.

The device of this invention prevents currents above frequencies of suchas about 10 lc'locycles per second from flowing through a squib, withthe result that the possibility of accidental firing of a squib byenvironmental radiation is virtually eliminated. The devices of thisinvention are particularly effective in protecting a squib located inthe vicinity of transmitters broadcasting at radar or radio frequencies,such as occurs on the decks of aircraft carriers and the like.

It is an object of this invention to provide a firing squib andprotective device therefor wherein the squib is protected fromaccidental actuation by environmental electromagnetic radiation.

It is another object of this invention to provide an integral squib andprotector therefor which may be used in substantially any application inwhich a firing squib is now used.

It is another object of this invention to provide a rugged circuitelement which is unaffected by impact acceleration, temperature orcorrosive conditions.

Other objects will become apparent from the drawings and from thefollowing detailed description in which it is intended to illustrate theapplicability of the invention without thereby limiting its scope toless than that of all equivalents which will be apparent to one skilledin the art. In the drawings like reference numerals refer to like partsand:

FIGURE 1 is a cross-sectional elevation of one embodiment of the deviceof this invention;

FIGURE 2 is a cross-sectional elevation of another embodiment of thisinvention wherein a primary Winding is remotely disposed from thesecondary winding;

FIGURE 3 is a cross-sectional elevation of another embodiment of thedevice of FIGURE 3, but wherein the primary circuit is configured toprovide magnetic flux of high density linking the secondary winding;

FIGURE 4 is a cut away of a perspective view of a coaxially woundtoroidally configured device of this invention;

FIGURE 5 is a crosssectional elevation of another embodiment of thisinvention wherein a unitary shield is shown;

FIGURE 6 is a cross-sectional perspective view of another embodiment ofthis invention wherein a shielded primary winding is shown;

FIGURE 7 is an exploded perspective of a rocket provided with a deviceof this invention;

ildhfiilh Patented Evept. I5, l fi it FIGURE 8 is a cross-sectionalelevation of another embodiment of this invention wherein an integralcore and shield member is shown;

FIGURE 9 is a cross-sectional elevation of a device of this inventionwherein a primary and secondary circuit winding of an inductor aredisposed adjacent opposite surfaces of a shield member;

FIGURE 10 is a cross-sectional elevation of another embodiment of thisinvention wherein shield means comprises one electric connection betweenthe secondary circuit and the squib;

FIGURE 11 is a graph of operating data of one device of this inventionin comparison with a device which is not in accordance with theinvention;

FIGURE 12 is a graph of operating data of devices of this inventionwherein shield thickness, core diameter, and ratio of primary tosecondary turns are changed;

FIGURE 13 is a graph of voltage magnitude of a direct current pulse in adevice of this invention;

FIGURE 14 is a graph of secondary circuit voltage when multiple layersof parallelly connected primary circuit windings are provided.

In FIGURE 1 is shown device 10 comprising core 11 of ferromagneticmaterial such as iron with secondary circuit windings l2 wound thereon.Lead wires 13 connect the ends of windings 12 to posts 14 of squib 15.Posts 14 of squib 15 pass through insulating plug 16 and are connectedto the ends of bridgewire 17. A thermally ignitable charge of material18 is disposed about bridgewire 17. Booster charge 19 is providedimmediately adjacent material 18, and main charge 20 fills the remainingvolume of squib 15. Charges 19 and 20 are eX- plosive materials such asblack powder or the like. The construction of squib 15 is conventionaland comprises no part of this invention. Shield 21 is a closedconductive container which encloses windings 12 and lead wires 13 andthe portion of posts 14 exterior to squib 15. Container 21' enclosingthe ignitable powder of squib 15 is, in a preferred embodiment of thisinvention, conductive, and is tightly and hermetically sealed to shield21 so that a continuous conductive enclosure is provided by the twomembers. Primary circuit windings 22 are wound on the outside of shield21. Secondary circuit insulated winding 12 and shield 21 are separatedby dielectric material 23. Material 23 may comprise any dielectric andpreferably comprises synthetic resin in which core 11, windings 12thereon, and lead wires 13 can be potted in operable manner. In apreferred embodiment of this invention, container 21 and shield 21comprises a non-magnetic metal or alloy such as copper, tin, zinc,aluminum or magnesium or may comprise paramagnetic or ferromagneticmaterial such as steel, nickel, or other operable conductive material,including non-metallic materials such as carbon, graphite, etc.

A squib as used herein is conventional in construction and may be forexample a military version which is 0.437 inch long and 0.271 inch indiameter. Device 16 is preferably not more than three inches in lengthand 1 /2 inches in diameter. As is aparent, device 10 may be made muchsmaller in dimension than the preferred limiting size. In operation,electrical current flowing from a supply source such as abattery-powered or generatorpowered source flows through conductors 29and primary circuit coil 22. A changing magnetic field is created by achange in current in the coil so that an initial current pulseincreasing in amperage generates an expanding magnetic field which cutsacross secondary circuit windings l2 and induces a voltage differencetherein. Current flows in winding 12 and leads 13 through bridgewire 17to ignite initiator charge 18 and detonate the squib. Current flow ofthe order of a few tenths of an ampere in bridgewire 1'7 will besufficient to ignite charge 18 when the bridgewire is of suchcomposition and dimension as to become sufficiently heated to ignite theexplosive charge by the current flow. Corrosion-resistant material suchas platinum-iridium alloy is preferred for the bridgewire. The initialcurrent flow in winding 22 desirably should be sufficient to operablyignite suib 15 so as to enable either direct or alternating current tobe used in the energizing circuit. Primary circuit potentials of 28volts are commonly provided in aircraft armament systems, and it isdesirable in such systems to provide a safety factor so that a squidwill fire when lesser potentials such as or 12 volts or less aredeveloped in the primary circuit. In the devices shown herein, such asin device 1d of FIGURE 1, voltages may be stepped up or stepped down asdesired by varying the ratio of turns in the windings of primary andsecondary circuits, as will be apparent. Also, the duration of currentflow in the secondary circuit when a step-wave of direct current issupplied to the primary circuit may be Varied by the ratio and number ofturns in the windings of primary and secondary circuits.

The windings of the primary and the secondary circuits in device 10 ofFIGURE 1 are disposed in superimposed concentric relation, but otherarrangements which provide sufiicient changing magnetic flux through thewinding of the secondary circuit to induce the energy necessary toignite squib 15 may also be utilized. Shield 21 comprises a container ofconductive material which encloses the secondary circuit elements. Theprovision of shield 21 operably prevents high-frequency current of amagnitude sufficient to fire squib 15 from being induced in secondarywinding 12 and leads 13. Energy transfer from spurious radiation ofradio or radar frequencies between the primary and secondary circuits isminimized by the interdisposition of the metal shield between theprimary and secondary windings. High-frequency currents which areinduced in the primary circuit are expended as eddy currents in theshield in a surface layer adjacent the outermost surface of the shield.The eddy current flow is expended by energy degradation into heat whichis readily dissipated by conduction and radiation from the shield.Energy transfer from currents of lesser frequency is operably providedin the firing circuit for igniting the squib, because the thickness ofthe shield is much less than the skin thickness for current of thatfrequency. Skin thickness 6 is that linear dimension transverse tocurrent flow in which current or field penetrating into a conductor manytimes 5 in thickness will decrease to times its magnitude at the surfaceof the conductor wherein e is the base of natural logarithms. The skinthickness in meters is given as where f=frequency in c.p.s., apermeability of the metal in h/m, a=conductivity of the metal in mhos/m.The factors to be considered in determining optimum shield thickness arethe frequencies of the current to be passed and of the current to besuppressed, as well as the permeability and conductivity of the metal orother conductive material which is used for the shield. It will beapparent from the formulas presented that the amplitude of any givenhigh-frequency current in the primary circuit may be suppressed in thesecondary circuit to any degree desired virtually to total eliminationby proper selection of shield composition and thickness. Conversely, theamplitude of any given low-frequency current can be passed by the shieldvirtually without attenuation if desired. Although other factorsinfluence the efficiency of the energy transfer by inductive couplingherein, the shield is the primary frequency-dependent variable. Thus, arelatively narrow cut-off frequency range is provided by a device ofthis invention, whereas a much broader cut-off frequency range isprovided by an unshielded device. By proper selection of a shield anygiven high-frequency current in the primary circuit may be quenched orattenuated in any degree for safety in the inductively coupled secondarycircuit without material- 1y attenuating energy transfer by lowerfrequency currents. Thus, any safety factor may be provided as desiredto insure that high-frequency currents of operable amplitude are notinduced in the secondary circuit.

The necessary embodiment common to all of the devices of this inventionis that a continuous shield is pro vided for the secondary circuitelements including the secondary circuit windings, the squib, and thelead Wires therebetween which operably shield and enclose such membersfrom the effects of high frequency electro-rnagnetic radiation in theimmediate environment of the shield.

In FIGURE 2 is shown another embodiment of the invention wherein primarycircuit windings 4th on core 41 are remotely disposed from secondarycircuit windings 42 and conductive shield 43. Shield 43 comprises asealed enclosure which is electrically connected to the enclosure ofsquib 15. The enclosure of squib device 15 is electrically conductive.The device shown in FIGURE 2 operates in substantially identical mannerto the device of FIGURE 1 by a relatively low frequency energizingcurrent flowing in primary circuit windings 4-0 producing a changingmagnetic flux which penetrates shield 4-3 and secondary circuit windings42 and induces a voltage in the secondary circuit across the bridgewireof the squib sufficient to detonate the squib. The initial pulse ofcurrent which flows in the secondary circuit is sufficient to ignite thesquib so that the device is operable with either direct current oralternating current, the magnitude of the voltage in the secondarycircuit being a function of the rate of change of current flow in theprimary circuit. The more nearly instantaneous the change in the primarycircuit, the greater is the voltage induced in secondary circuit andacross bridgewire 17 in squib 15. It is to be understood that thedisposition of primary circuit windings and secondary circuit windingsin the devices of this invention are not criti al to the invention, butare material only to the production of operable energy in the secondarycircuit to detonate the squib.

In FIGURE 3 is shown another embodiment of the in vention whereinprimary circuit windings are wound on horseshoe-configured core 51.Conducting shield 43 enclosing secondary circuit windings 42 on core 44is disposed between the opposed end extremities of core 51. Squib device15 is sealed to enclosure 43 and electrical connection is establishedbetween enclosure 4-3 and the outer case of squib 15. The geometry ofwindings Sit on core 51 provides a greater flux density through core 44of secondary windings 4-2 than does the asymmetrically disposed primarycircuit of FIGURE 2. The utilization of magnetic fiux in the device ofFIGURE 3 is superior to that of the device of FIGURE 2 and theconfiguration is therefore preferred to the configuration of FIGURE 2.

In FIGURE 4 is shown another embodiment of the invention wherein thewindings of the primary circuit and secondary circuit are coaxiallywound in a torroidal-configuration. Core 55 is of circular configurationand comprises ferro-magnetic materials such as soft iron or the like,either laminated or non-laminated as may be desired. Secondary circuitwindings se of insulated wire are helically wound on core 55substantially entirely around the torroid. Conductive layer 57 isprovided concentrically around core 55 and enclosing secondary circuitwindings 56 to shield the secondary circuit winding fromelectro-magnetic radiation which would otherwise induce a voltage withinsecondary windings 56. Primary circuit windings 68 are helically woundconcentrically upon layer 57 to provide a device similar ,a D inoperation to the devices heretofore described. Sec ondary windings 56may be operably connected to squib 15 in operable manner with the outerenclosure of squib 15 and conducting layer 57 being connected to providea continuous electric current path through the squib enclosure and layer57 in the device. The device shown in FIGURE 4 is of torroidalconfiguration but may be of substantially square or rectangularconfiguration or any other operable configuration which entirely shieldsthe secondary circuit elements, including windings 56, squib 15 and atleast one lead from the windings to the squib.

In FIGURE 5 is shown another embodiment of the invention wherein squib15 is disposed interior to shield 60 together with secondary circuitwinding 61, and core 62. The secondary circuit windings, squib device,and secondary circuit core are substantially similar to the device ofFIGURE 1. Primary circuit windings 63 are provided in coaxialarrangement with secondary circuit windings 61 and external to shield 60and wound thereon. In all cases wherein a primary circuit element orsecondary circuit element is wound on a conducting member the windingsare operably insulated to prevent current leakage from the circuit. Itis apparent that the device of FIGURE 5 can only be used in thoseapplications wherein a squib may be operably disposed apart from thecharge to be ignited or when that charge may also be enclosed by shield60. For this reason, the configuration shown in FIGURE 5 is not apreferred embodiment of this invention.

In FIGURE 6 is shown another embodiment of this invention wherein doublewalled shield 74 is provided around secondary circuit winding 70 on core72 and primary circuit winding 73 coaxially disposed thereabout in anannular space between walls of double-walled annular shield 74, asshown. Shield 74 protects the windings of both the primary and secondarycircuits from environmental electro-magnetic radiation and therebydoubly protects the squib from spuriously induced currents. That portionof the shield which encloses the primary circuit winding has no effecton the energy transfer between circuits, and the design considerationsare similar to those heretofore described; however, a lesser safetyfactor may be justified by providing a shielded primary circuit winding.A squib 15 may be connected to windings 70 and shield 74 in the mannershown.

In FIGURE 7 is shown a 2.75 inch rocket in exploded view whereinprojectile 27 comprises the foremost portion of the rocket, andtailpiece 81 comprises the rearmost portion of the rocket. Pins 82,tailpiece 81, provide flight-stabilizing means for the rocket. Bodymember 26 contains the propulsive charge for the rocket and is providedwith resonance rod 25 in the fore-end of which device and squib ofFIGURE 1 are operably disposed. Device 10 may be any other operablemeans heretofore described. Projectile 27 comprises a warhead inoperable manner.

In FIGURE 8 is shown another embodiment of the invention wherein primarycircuit windings 85 are disposed coaxially with secondary circuitwindings 86 and wherein core 87 is electrically conductive and ispreferably ferromagnetic, for most efiicient operation of the device asan inductor. Dielectric material such as synthetic resin or the like maybe provided in the annular space within the member 87 if desired, but isnot necessary.

In FIGURE 9 is shown another embodiment of this invention whereinprimary circuit windings 99 are wound on the outside of conductivecasing Q1 and secondary circuit windings 92 are wound on the inside ofcasing 51. The secondary windings are operably connected to squib 15 byelectric leads E3. A core is not provided for the secondary circuit inthis embodiment of the invention and therefore the embodiment is not apreferred embodiment of the invention inductive efficiency of the devicebeing less than the embodiments wherein a ferromagnetic core is providedfor the secondary circuit.

In FIGURE 10 is shown another embodiment of the invention wherein shield95 is provided with primary circuit windings 96 of insulated conductorwound thereon. Shield 95 is filled with di-electric material such asphenolic thermoset synthetic resin, polymethylmethacrylate, polyethyleneor other operable material. Secondary circuit elements comprisingsecondary circuit winding 93, lead wire 99, lead wire 100, and core 101are potted in material 97 in operable manner. Conductive container 102is operably sealed to shield 95 by conductive resinous material 103.Material 103 may comprise a conventional synthetic resin which isadmixed with metal particles or the like to be rendered conductive, suchmaterial being commercially available and comprising no part of thisinvention. Lead wire 100 is connected to the shielding enclosurecomprising shield $5, container m2, and material 103. Lead wire 9? isinsulated from the shielding enclosure and connects to squib 15 asshown. Lead wire 10- 1- from squib 15 is connected to container 102 toprovide a closed circuit through the container, material 103, and leadwire 100 to secondary winding 98.

The following examples illustrate the applicability of this invention toa conventional type of squib for military armaments. As used hereinsquib means the sealed, unitized explosive charge, bridge wire and postsor connectable lead wires therefore and does not include windings.

Example 1 Device 1(a) was constructed as shown in FIGURE 1 above. Asquib of a type usable with the device has the following operatingcharacteristics;

Resistance of squib circuit: 0.7-1.3 ohms Squib circuit test current: 10ma.

100% inoperable firing current: 0.2 amp.

100% etfective firing current: 0.3 amp.

Recommended firing current: 1.5 amp.

Electro-static sensitivity: 52,000 ergs from a 400 mrnfd.

condenser Bridge wire: 0.001 inch platinum-iridium alloy Wire Lead wirelengths: Vary between 1% inches and 20 /2 inches, according to modelWeight: 1.25 grams to 5.25 grams, depending on lead wire length.

Enclosure 2]. comprised 0.014 inch thick copper sheet configured as acylinder with a 0.5 inch outside diameter. Primary winding 22 comprisedone layer of No. 24 enamelled copper wire close wound for 1.5 inchesaxial length. Secondary winding 12 comprised wires having the samenumber of turns as the primary winding and wound on a 0.38 inch diametercold drawn iron core.

Device I(b) comprised a device similar to device 1(a), but whereincardboard was substituted for copper in shield 21.

To standardize experimental procedure, a calibrated one-ohm load wassubstituted for the squib in all tests. Open circuit and closed circuittests were performed. The results of the tests are graphed in FIGURE 11wherein the ratio of the secondary circuit voltage (V to the primarycircuit voltage (V is plotted against the frequency of the chargingcurrent in the primary circuit. The curves marked 0.c. depict opencircuit tests and the curves marked 10 depict closed circuit tests.Closed line curves show results obtained with device 1(a) and brokenline curves show results obtained with device I(b).

It will be observed that at frequencies of 10 kc. and greater thevoltage induced in the secondary circuit of device 1(a) is attenuatedwith respect to the voltage in the primary circuit thereof more stronglythan in device 1(1)).

Example II A device substantially as shown in FIGURE 1 comprising onelayer of closely spaced windings of No. 24 enamelled copper wire havinga diameter of 0.021 inch for approximately 1.50 inches axial length wasprovided. The dimensions of shield 21 were 0.50 inch outside diameterand 0.470 inch inside diameter with a 0.015 inch thick copper wall. Theshield was configured with one closed end and one closable end. Thesecondary winding comprised six concentric layers in series of No. 24enamelled copper wire wound on a cold drawn iron core of 0.185 inchdiameter for an axial length of 1.25 inches. Each layer comprised thesame number of turns as primary windings.

Example III Primary circuit: same as Example 11.

Secondary circuit: same as Example II.

Shield: copper of 0.50 inch outside diameter and 0.44 inch insidediameter; wall thickness of 0.03 inch.

Example IV Primary circuit: same as Example II.

Secondary circuit: same as Example 11'.

Shield substitute: Polymethylmethacrylate of 0.50 inch outside diameterand .030 inch wall thickness.

Example V Primary circuit: same as Example 11.

Secondary circuit: same as Example I} except four layers in series ofwire were provided on core of 0.295 inch diameter.

Shield: same as Example 11.

Example VI Primary circuit: same as Example 11.

Secondary circuit: same as Example 11 except two layers in series ofwire were provided on a core of 0.380 inch diameter.

Shield: same as Example 11.

Example VII Primary circuit: same as Example 1!.

Secondary circuit: same as Example Tl, except 0.215 inch diameter corewas provided.

Shield: same as Example TI.

The results of tests with the devices of Examples 1i- VII when providedwith one-ohm loads in the secondary circuit are shown in FIGURE 12. Theratio of voltage in the secondary circuit (V to voltage in the primarycircuit (V is plotted versus frequency of the current in the primarycircuit.

A difference in the number of layers of windings connected in series inthe secondary circuit, as shown in the curves for the devices ofExamples H, V, and VI in EEG- URE 12, wherein 6, 4, and 2 layers,respectively, of series connected windings are provided, produces theresults shown. The voltage-time response of the secondary circuit for6-volt direct current in the primary circuit is shown in FIGURE 13 forthe devices of Examples V11, V, and VI. The number of layers of windingsis relatively non-critical to the invention herein set forth.

The ratio of voltage in the secondary circuit to voltage in the primarycircuit is materially affected by the shield thickness and is decreasedin proportion to the increase in the thickness of a shield, as shown inthe curves for the devices of Examples 11, Ill, and IV. The shieldthickness is therefore a material factor in the selection of a devicefor use in a particular application. As shown by the curve in FIGURE 12,for Example IV, a non-conducting material in place of the metal shielddoes not materially influence the pick-up of current in the squib and isnot operable herein because adequate certainty of attenuation is notprovided.

The eifect produced by changing the diameter of the core in which thesecondary winding is wound is shown on FIGURE 14 for the curves ofExamples II and VII to be relatively minor, although not negligible.

All of the devices described and illustrated provide substantiallysimilar characteristic response to change in highfrequency radiation.

Example VIII The device 1(a) of Example I was provided in separate testswith 1, 2, and 3 layers of parallel connected primary circuit windings.The secondary circuit was provided with a one-ohm load. A 6-volt batterywas connected to the primary circuit by means of a mercury switch. Thepulses produced by flow of direct current in the 1, 2, and 3 parallelconnected primary circuit layers are shown by curves u a b 11 c c ofFIGURE 14, wherein a, b, c relate to 1, 2, and 3 layers, respectively ofprimary windings and subscripts p and s relate to primary and secondarycircuits respectively.

While certain modifications and embodiments of the invention have beendescribed, it is of course to be understood that there are a greatnumber of variations which will suggest themselves to anyone familiarwith the subject matter thereof, and it is to be distinctly understoodthat this invention should not be limited except by such limitations asare clearly imposed in the appended claims.

I claim:

1. Means to initiate a rapid self-sustaining chemical reactioncomprising, in combination with a member which. is heated by passingelectric current through the member and reactant substances adjacentsaid member:

a secondary Winding having its ends electrically connected respectivelyby conductive means to the ends of said member, a primary windinginductively coupled to said secondary winding, means to connect the endsof said primary winding to a source to provide at least one pulse ofelectrical current in said primary winding to provide a field ofchanging magnetic flux which links said secondary winding,

a single core of magnetically soft metal, said core providing a core forsaid secondary winding and also for said primary Winding, said coreextending through both of said windings, said core being ofsubstantially constant cross-sectional area and shape throughout theportion of its length which extends through said windings,

said core being unshunted around said primary windfirst supportingmeans, said secondary winding wound on said supporting means, secondsupporting means rigidly and securely attached, as by potting, to saidfirst supporting means, said primary winding wound on said secondsupporting means,

characterized by an electromagnetic shield interposed between saidprimary and secondary windings,

said shield entirely electrically enclosing the secondary Winding, themember, at least the entirety of one electrical conductor extending fromsaid secondary winding to said member, and said reactant substances,

said shield devoid of electrical conductors extending therethrough,

said shield constituting a shorted turn,

said shield essentially devoid of electrical apertures therein.

2. The device of claim 1 further characterized by means which provide aclosed path through magnetic material for the flux which links saidprimary and secondary windings.

3. The device of claim 2 wherein said reactants constitute an explosivemixture.

4. The device of claim 2 wherein said reactants react to produce abrilliant flash of light.

5. The device of claim 2 wherein the means which connect the ends of thesecondary winding to the ends of the filament are insulated from theshield.

6. The device of claim 2 further characterized by said core beingtoroidal.

7. The device of claim 1 wherein said reactants constitute an explosivemixture.

8. The device of claim 1 wherein said reactants react to produce abrilliant flash of light.

9. The device of claim 1 wherein the means which connect the ends of thesecondary winding to the ends of the filament are insulated from theshield.

10. The device of claim 1 wherein a core of magnetic material isprovided for said secondary winding and at least a portion of said coreprovides a portion of said shield.

11. The device of claim 1 wherein a core of magnetic material isprovided for said secondary winding and said core is electricallyinsulated from said shield.

12. Means to initiate a rapid self-sustaining chemical reactioncomprising, in combination with a member which is heated by passingelectric current through the member and reactant substances adjacentsaid member:

a secondary winding having its ends electrically connected respectivelyby conductive means to the ends or" said member, a primary windinginductively coupled to said secondary winding, means to connect the endsof said primary winding to a source to provide at least one pulse ofelectrical current in said primary winding to provide a field ofchanging magnetic flux which links said secondary winding,

a single core of magnetically soft metal, said core providing a core forsaid secondary winding and also for said primary winding, said coreextending through both of said windings, said core being ofsubstantially constant cross-sectional area and shape throughout theportion of its length which extends through said windings,

said core being unshunted around said primary windfirst supportingmeans, said secondary winding wound on said supporting means, secondsupporting means rigidly and securely attached, as by potting, to saidfirst supporting means, said primary winding wound on said secondsupporting means,

characterized by an electromagnetic shield interposed between saidprimary and secondary windings,

said shield entirely electrically enclosing the secondary winding, themember, at least the entirety of one electrical conductor extending fromsaid secondary winding to said member, and said reactant substances,

said shield devoid of electrical conductors extending therethrough,

said shield constituting a shorted turn,

said shield being altogether continuous throughout and totally devoid ofany electrically effective apertures therein.

13. The device of claim 12 further characterized by means which providea closed path through magnetic material for the flux which links saidprimary and secondary windings.

14. The device of claim 13 wherein the means which connect the ends ofthe secondary winding to the ends of the filament are insulated from theshield.

15. The device of claim 13 further characterized by said core beingtoroidal.

16. The device of claim 12 wherein the means which connect the ends ofthe secondary winding to the ends of the filament are insulated from theshield.

17. The device of claim 12 wherein a core of magnetic material isprovided for said secondary winding and at least a portion of said coreprovides a portion of said shield.

18. The device of claim 12 wherein a core of magnetic material isprovided for said secondary winding and said core is electricallyinsulated from said shield.

References Cited in the file of this patent UNITED STATES PATENTS1,320,980 Bowman Nov. 4, 1919 1,689,929 Osborne Oct. 30, 1928 2,459,854Swift Jan. 25, 1949 2,628,342 Taylor Feb. 10, 1953 2,640,417 Bjork et alJune 2, 1953 2,678,413 Adler et al. May 11, 1954 2,905,915 Harris Sept.22, 1959 2,918,001 Alford Dec. 22, 1959 2,921,522 Apstein Jan. 19, 19602,948,871 Craige Aug. 9, 1960 3,038,384 Gaugler June 12, 1962

1. MEANS TO INITIATE A RAPID SELF-SUSTAINING CHEMICAL REACTIONCOMPRISING, IN COMBINATION WITH A MEMBER WHICH IS HEATED BY PASSINGELECTRIC CURRENT THROUGH THE MEMBER AND REACTANT SUBSTANCES ADJACENTSAID MEMBER: A SECONDARY WINDING HAVING ITS ENDS ELECTRICALLY CONNECTEDRESPECTIVELY BY CONDUCTIVE MEANS TO THE ENDS OF SAID MEMBER, A PRIMARYWINDING INDUCTIVELY COUPLED TO SAID SECONDARY WINDING, MEANS TO CNNECTTHE ENDS OF SAID PRMARY WINDING TO A SOURVE TO PROVIDE AT LEAST ONEPULSE OF ELECTRICAL CURRENT IN SAID PRIMARY WINDING TO PROVIDE A FIELDOF CHANGING MAGNETIC FLUX WHICH LINKS SAID SECONDARY WINDING, A SINGLECORE OF MAGNETICALLY SOFT METAL, SAID VORE PROVIDING A CORE FOR SAIDSECONDARY WINDING AND ALSO FOR SAID PRIMARY WINDING, SAID CORE EXTENDINGTHROUGH BOTH OF SAID WINDINGS, SAID CORE BEING OF SUBSTANTIALLY CONSTANTCROSS-SECTIONAL AREA AND SHAPE THROUGHOUT THE PORTION OF ITS LENGTHWHCIH EXTENDS THROUGH SAID WINDINGS, SAID CORE BEING UNSHUNTED AROUNDSAID PRIMARY WINDING; FIRST SUPPORTING MEANS, SAID SECONDARY WINDINGWOUND ON SAID SUPPORTING MEANS, SECOND SUPPORTING MEANS RIGIDLY ANDSECURELY ATTACHED, AS BY POTTING, TO SAID FIRST SUPPORTING MEANS, SAIDPRIMARY WINDING WOUND ON SAID SECOND SUPPORTING MEANS, CHARACTERIZED BYAN ELECTROMAGNETIC SHIELD INTERPOSED BETWEEN SAID PRIMARY AND SECONDARYWINDINGS, SAID SHIELD ENTIRELYH ELECTRICALLY ENCLOSING THE SECONDARYWINDING, THE MEMBER, AT LEAST THE ENTIRETY OF ONE ELECTRICAL CONDUCTOREXGENDING FROM SAID SECONDARY WINDING TO SAID MEMBER, AND SAID REACTANTSUBSTANCES, SAID SHIELD DEVOID OF ELECTRICAL CONDUCTORS EXTENDINGTHERETHROUGH, SAID SHIELD CONSTITUTING A SHORTED TURN, SAID SHIELDESSENTIALLY DEVOID OF ELECTRICAL APERTURES THEREIN.